862doi:10.1017/S1431927618004804

Microsc. Microanal. 24 (Suppl 1), 2018© Microscopy Society of America 2018

Low-Energy Argon Broad Ion Beam and Narrow Ion Beam Milling of In Situ Lift-Out FIB Specimens M.J. Campin, C.S. Bonifacio and P.E. Fischione E.A. Fischione Instruments, Inc., Export, PA USA Focused ion beam (FIB) tools are commonly used to prepare transmission electron microscopy (TEM) specimens due to the site specificity and accuracy of specimen thinning and extraction that the tools provide [1,2]. However, it has been shown that milling with Ga ions results in a damage layer on each side of the TEM lamella, the thickness of which is proportional to the voltage of the ion beam [3]. The damage layer reduces the signal-to-noise ratio and contrast in TEM and scanning transmission electron microscopy (STEM) images, which negatively affects the ability to perform high resolution imaging and analysis. To minimize this, a low-energy Ga ion beam (≤ 2 keV) can be used for final thinning of the TEM lamella [3-4]. Post-FIB milling with a low-energy Ar ion beam can then be done to remove the damage layer and Ga implantation [4-6]. When used in combination, specimens free of surface damage and Ga implantation can be created [7]. In this work, TEM lamellae from a bare semiconductor wafer were prepared by a dual-beam FIB using the in situ lift-out technique. Specimens were milled at 30 keV to a targeted thickness of ~100 nm and then final cleaned at 5 keV to reduce the thickness of the damage layer. Post-FIB thinning was done at low-energy (≤ 0.5 keV) with either an argon broad ion beam (~1 to 2 mm diameter) milling tool or an argon narrow ion beam (~1 µm diameter) milling tool, the latter of which rasters the ion beam within a milling box placed over the lamella. To minimize milling of the protective capping layer, the maximum milling angles of ±10˚ were used when milling with the broad ion beam. For consistency, the same angles were used when milling with the narrow ion beam. High resolution TEM (HRTEM) imaging and energy-dispersive X-ray spectroscopy (EDS) analysis were performed with a field emission gun TEM/STEM at 200 kV to determine the reduction in amorphous damage and Ga implantation. HRTEM images taken after broad ion beam milling (Figure 1) show variations in contrast across the images and the corresponding fast Fourier transforms (FFTs) show that a diffuse halo is still present. This indicates that some of the damage layer remained after milling. In contrast, HRTEM images taken after narrow ion beam milling (Figure 2) show very uniform contrast across the images and the diffuse halo in the corresponding FFTs is gone. This indicates that the damage layer was removed. EDS analysis (Figure 3) showed that broad ion beam milling removed some of the Ga, but that narrow ion beam milling removed all of the Ga. Quantitative analysis of sample thickness and sputtering of grid material onto the TEM lamella at various milling voltages up to 0.5 keV is in progress.

References: [1] LA Giannuzzi and FA Stevie, Micron 30 (1999), p. 197. [2] J Mayer et al., MRS Bulletin. 30 (2007), p. 400. [3] R Kelley et al., Microscopy and Microanalysis 19 (Suppl 2) (2013) p. 862. [4] LA Giannuzzi et al., Microscopy and Microanalysis 11 (Suppl. 2) (2005) p. 828. [5] M Schaffer et al., Ultramicroscopy 114 (2012) p. 62. [6] M Mitome, Journal of Electron Microscopy 62(2) (2013) p. 321. [7] P Nowakowski et al., Microscopy and Microanalysis 23 (Suppl. 2) (2017) p. 300.

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1a 1b

Figure 1. HRTEM images acquired before (a) and after (b) broad ion beam milling show a reduction in contrast variation. Corresponding FFTs show a reduction in the amorphous layer thickness. 2a 2b

Figure 2. HRTEM images acquired before (a) and after (b) narrow ion beam milling show complete contrast uniformity after milling. Corresponding FFTs show amorphous layer removal.

Figure 3: EDS spectra acquired from the silicon after FIB preparation, after broad ion beam milling and after narrow ion beam milling show complete removal of gallium after narrow ion beam milling.